Pulse regenerator circuit



DDPPDDP 2 SHEETS- SHEET l W. H. BLISS Dec. 4, 1951 PULSE REGENERATOR cRCUIT Filed April 28, 1945 IN VEN TOR. Mmm lidf BY Mg f A T TORNE Y.

Dec. 4, 1951 W H, BLISS 2,577,444

PULSE REGENERATOR CIRCUIT Filed April 28, 1945 2 SHEETS- SHEET 2 l [VEN TOR.

@fren A TTURNE Y.

Patented Dec. 4, 1,951 ,1

PULSE REGENERATOR CIRCUIT Warren H. Bliss, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application April 28, 1945,l Serial No. 590,822

12 Claims.

The present invention relates to a system for converting relatively short keyed pulses into conventionally longer duration telegraph keyed characters, such as dots and dashes.

One method of radio telegraph communication consists in converting, at the transmitting station, conventional dots and dashes of the telegraph code into shorter duration equal length pulses of radio frequency energy which may be keyed in short and long space groups, and these shorter pulses radiated toward the remote radio receiving station. Reference is herein made to my United States Patent No. 2,365,450, granted December 19, 1944, and to my copending application Serial No. 491,088, filed June 17, 1943, now United States Patent No. 2,457,974 of January 4, 1949, for a description of this type of system in which the transmitted short spaced pulses are regenerated or reconstructed (rebuilt) at the receiving terminal and converted to again form the conventionally longer code characters.

The present invention comprises an improved pulse regenerator system for use in connection with a radio receiving station for transforming theA received relatively short pulses into the usual or conventionally longer duration code characters, suchas dots and dashes, so that they may be recorded for transcribing purposes or applied to another circuit for further transmission. 'I'his is eiected in accordance with the invention by means of a pair of monostable multivibrator or self-restoring electron discharge device trigger circuits, one multivibrator or trigger circuit of which controls the other multivibrator or trigger circuit, and each of which includes two multi-electrode structures. One trigger circuit supplies output pulses which occur in the intervals between the output pulses from the other trigger circuit, and the time constants of these trigger circuits have such values that the combined outputs of both trigger circuits simulate the desired longer duration conventional code characters.

, Advantages of the present invention overv the telegraph regenerative systems described in my" caused by fading, has no influence on the operation of the pulse generator; and thesystem f the Z invention will also handle conventional ldot and dash code characters transmitted by radio and remove noise and `fading effects.

A more detailed description of the invention follows in conjunction with drawings, wherein:

Fig. 1 illustrates circuit details of the pulse regenerator of the invention, and

Fig. 2 illustrates a series of wave forms given in explanation of the operation of the system of Fig. 1.

Referring to Fig. 1' in more detail, there is shown a radio telegraph receiver REC connected to an antenna A. The output of this receiver is connected to a pulse regenerator constituting the invention. Receiver REC and antenna A are representative of any suitable apparatus for performing their intended functions, such as a diversity receiving system in which a plurality of antennas are connected to the inputs of a plurality of superheterodyne receivers whose outputs are combined. The pulse reproducer in-'. cludes four gaseous tetrode tubes 3, I3, '20 and 25 and a dual diode vacuum tube 29. Tetrodes 3 and I3 operate together in an alternate conduction, monostable multivibrator, or self-restoring trigger circuit; while tetrodes 29 and 25 operate similarly in another monostable multivibrator or self-restoring trigger circuit. The circuit of tetrodes 3 and I3 may be called a reshaper be-v cause the tetrodes convert the shape of an input pulse applied to the grid of tube 3 into a substantially square or rectangular pulse, irrespective of the shape of the received initiating pulse; Tetrode 3 has a resistor 4 connected in series with its anode circuit, and alresistor 6 connected in its cathode circuit. The control grid of this tetrode is connected to the output of the receiver REC through a series connected grid resistor 2 and an input potentiometer I. Adjustable bias voltagev is supplied to the screen grid of tetrode 3 by means of battery 9 andpotentiometer 'I. Tetrode I3 has an anode resistor I2 and a cathode resistor I4. The anode of tetrode 3 is connected to the anode of tetrode I3vthrough a commutatingj condenser 8, and to the control grid of tetrode I3 through condenser l The anode resistors 4 and I2 are connected to the B+ anode supply. A

variable resistor II connects the control grid or tetrode I3 to ground. The screen grid of tube I3 is also., grounded. Cathode resistors 6 and I4 pulse wave form obtained from the reshaper or trigger circuit. Normally' (with no signal coming inl.' gas tube 3 is non-conducting while gas tube I 3 is in a state of conduction. The application of a signal or initiating pulse of positive polarity and of predetermined magnitude to the control grid of tube 3 will cause this tube to conduct, as a result of which tube I3 Will cease conducting by virtue of the commutating action of condenser 8. Putting it in other words, an initiating pulse or trigger willnr'efor triggenthereshaper 3, .I3 from theestable state-in which'tube' 3 is normally non-conducting and tube I3 normally conducting to the active or unstable state in which the cure.

rent passing conditions of theseftwo-tubesnare reversed. Output energy from the triggers 3 and" I3 may be taken from the cathodesof these tubes. trigger circuit is controlled by the values of condenser I and resistor II.

The operation of the trigger circuit 3, I3 may be better understood from the following explanation: With the stable.; condition of the selfrestoring trigger-circuit; tube I3 isconducting whileftube- 3 is non-conducting. For. purposes of explanation, thecathodes."ofthese two tubes maybe. assumed. tobe connected directly to groundf since-'the cathode resistors 16Vv and I4 are included :.solely forthe purpose of providing'an easy means ofideriving output signals. Because of) the low. voltage'- drop from v anode to cathode (about' 8 4volts'for: theseftype' 2050 tetrodes) the potential at the anode of tube I3 is almostVA at ground'potential (plus 8 volts, while 'B+ is,v say, +250.` volts) during: the f stable condition. This stable'conditionpersistsruntll the controlgrid of tube:3 Vis momentarily causedfto become.Y slightly moreipositive `than the'cut-oi; or "more strictly speaking, the firing point.

When tubel 3--isxtriggeredor fired, itinstantly starts to; conduct andfdrawfanode current,` the valueof which is controlled by resistor 4. When this conduction begins,- thev anode potential of tube3..dropsfrom'V +250 Vvolts (B+ supply value) to: about +8v volts (anode-to-cathode drop) and remainsrat that Value asv long 'as' conduction continues:in'tube 3.' At the-instant of this drop in potential, theconductionf'through tube- I3.- is s'nuiedV'out by theaction of condenser 8. A condenser-'is l:an: electrical device which offers opposition to suddenchangesain potential across itsterminalsfjust as achokecoil oiers opposition tosuddenchanges infcurrent throughit.Vv

.In the stable condition of the trigger circuit, theleft hand sideofcondenseris at +250 volts andtheright yhand sideis at +8 volts. When tube3-flres and.itslanodexdropsfrom +250 to +8 volts,` the left hand.- side of condenser 8. drops the same-amount. Since thev potential across condenser i3 cannot .change instantaneously, the right hand side ofcondenser 8 must also drop from wherever. it is (+8Y volts) to a value 250-8`=242' volts lower. Sincethe right hand sideof condenser 8'is connected to the anode of tube I3. the anode of'thistube'is driven (momentarily at leastl to a negative. value' of 242"-8`=234 volts.. Since conventional tubes cannot conduct when 'the' anode'is' negative with respect'tozthe cathode, tubeY [3 cease'sconductlon: Condenser 8 is usually quite small, for example 0.0015 afd., andneedlbe.only'large enough to hold the anodeoftube'l3 negative .long enough for deionization of the-gas in this tube. After this'switchingor exchange-of states of conduction ofthe. two tubes,` condenser 8 quickly assumes its new status-of potentials. Theleftside stays at +8 .volts f and` th`ef:rightu hand. side .rises tot', +250 volts." VThe :unstable /ori' active. state 0f The duration of theactiveperiodofithe:

, whichis the same potential as the left side of condenser 8'.

Whentube 3starts conduction, condenser I0 performsiin" a manner similar to condenser 8 but condenser I0 drives the control grid of tube .I3.to-a negative value far below the cut-oli or firing point. 'Since condenser I0 is much larger than condenser 8, it holds the control grid of tube I3 at a low negative value for an appreciablelength of time; Tube I3 is thusprevented from rering eventhough its anode has already recoveredfrom the action of condenser 8. After an interval of time determined by the size of condenser I0 and the size of resistor II, the potential of the lower side of condenser I0 does rise-to such a value that tube I3 reres, and condenser 8 acts to snuff out tube 3. The circuit. is .then in the stable state again.

Therscreenfgrid voltages on these tubes 3 and I3KA can be adjustedso as to set -the control grid ringpotential at any desired value over a wide rangeof conditions..v Inthe circuit-of Fig. 1, the screenY grid of tube His-operated at-ground potential -while the screengridof tube 3 is operated at anegative. value.

Fromthe above explanation it may be said that the functions of condensersl and.|0 arev entirely separate and independent. Condenser 8v acts as alswitch such that whenthe non-conducting tube starts` conducting .itswitches off the other tube. Condenser I0 holdstube I3 in a state of non-.conduction and together with resistor II controls the length .of vthe non-conduction period of tube I3.

A'Il'ie circuit connections for gas tetrodes 20 and 25are similar to those of'tubes 3 andA I3 with certain exceptions given below. Tubes 20 and 25-al`so comprise a self-restoring trigger circuit. Tetrode 20 is normally non-conducting, while tetrode 25.`is normally conducting. The input to the control grid of tube 20 is taken from the cathode of tube I3 by way of condenser I6 and resistors I 'l and I8, as shown. The screen grid oi'tetrode'20 is supplied with a ilxed bias from battery 22'by virtueof'a connection'to the negative terminal Voi this battery. The cathode of tetrode 25 is connected directly to ground. It should be noted thattheanodes of tubes 20 and 25 are connected together by way of commutating condenser 23,- and that the anode of tube 20 is also connected to the control grid of tube 25 by way of'condenser 24. Condenser 28 is a lay-pass condenser across the cathode resistor 2l to improve the rectangular pulse Wave form obtained from this cathode resistor. Resistors I0 and.v 26'indivi'dually connectthe anodes of'tubes 20 and 25 to.the.B+ anode supply. A variable resistor 21 connects thecontrol grid of tube 25 to ground. The duration ofthe active period of the trigger circuit 20, 25 is controlled by the values of condenser 24 and resistor 21..

Outputstfrom thextwo trigger circuits 3, I3 and. 20, ,25 4arei combinedinv adual Adiode' tube 29, and.

this corrxbned..output deliveredto leads 42 conhected across resistor 30 in the cathode circuits of the dual diode. It should be noted that one anode of the dual diode is connected by way of lead 50 to the upper terminal of cathode resistor 6, While the other anode of dual diode 29 is connected by way of lead 5| to the upper. terminal of cathode resistor 2 I A better understanding of the operation of the pulse regenerator of Fig. 1 may vbe had by referring to the graphical representation of Fig. 2. Fig. 2 shows a series of curves 40, 4| and 3| to 39, inclusive, representing voltage variations at various points designated hereinafter. Let us assume that the letter A in conventional code characters originates at the remotely located telegraph transmitting station. This letter A is shown in curve 40 and comprises -a dot, a space, and a dash. At the transmitter, two short pulses may, for example, be transmitted for each basic unit of the signal, so that the letter A will be converted into the form of two short equal length spaced pulses for the dot, and six short equal length spaced pulses for the dash. This is shown in curve 13|. Pulses of radio frequency energy corresponding to curve M are those radiated from the remote telegraph receiving station to be received by the system of Fig. 1. Curve or wave form 3| shows a typical incoming signal for the letter A as observed at the output terminals of the receiver REC and impressed upon the control grid of the tetrode 3. The iirst two spaced pulses 32 convey the dot of the code for the letter A and the second group of six spaced pulses 32 conveys the dash. Noise is shown between the received pulses and is represented by the jagged lines. The receiver output represented by curve 3| is applied to the control grid of tetrode 3 by means of potentiometer I.

Tetrode I3 is normally (no signal coming in) in a state of conduction, while tetrode 3 is normally non-conducting. When the system is properly adjusted, each incoming pulse (as it is applied to the control grid of tetrode 3) iires or triggers this tetrode into a state of conduction. When tetrode 3 starts conduction, tetrode I3 ceases conduction because `of the commutating action of interconnecting condenser 8. The conduction period of tetrode 3 is controlled by the values of condenser I0 and resistor II. When tetrode 3 res, the potential of the control grid of tetrode I3 is reduced to a low value which delays reconduction in tetrode I3 for a short interval of time. However, the time constant of the control circuit is adjusted so that tetrode |3 refires shortly before the arrival time of the next pulse. When tetrode I3 becomes conductive, tube 3 will become non-conductive. Line 34 shows the conduction versus time relation for tetrode 3. This line is also the wave form of voltage developed at the cathode oftetrode 3. Line 35, likewise, is the wave form of potential at the cathode of tetrode I3.

The bias on the screen grid of tetrode 3, adjusted by potentiometer 1, is the noise threshold control. This must be set at such a level that the noise peaks occurring between the signal pulses 32 of curve 3| will not cause false operation of tetrode 3. Dotted line 33 illustrates a suitable threshold setting, such that all pulses rising above this level will be utilized for ring gas tube 3, while lower amplitude noise peaks will It should also be noted that.

have no eiect. variationin signal pulse amplitude doesA not modify the circuit operations as long as this am; plitude Vdoes not drop below the theshold` level.

To fully reconstruct the signal; that is, to reproduce the original wave form of line`40, the short gaps in the wave form of line 34 between adjacent pulses of each dot or dash must be filled in. Tetrodes 20 and 25 accomplish this by operating in a manner similar to tetrodes 3 and I3; that is, one tube only of the pair is ina state of conduction at one time. Triggering pulses to fire tetrode 23 are derived from the output of tetrode I3 by means of a connection including condenser vI6 and resistor I8. The derived wave form is shown in line 3B and consists of a series of positive and negative pulses resulting from diierentiation of output of tetrode I3. The positive pulses 3l occur each time tetrode 3 terminates its conduction period and the pulses are applied to re tetrode 20 (tetrode 20 being initially nonconducting while tetrode 25 is conducting). The firing of tube 20 causes tube 25 to cease conducting for an interval of time depending upon the time constants of the circuit. Tetrode 25 resumes conduction a short interval of time after ceasing conduction, this interval being controlled by the values of condenser 2 and resistor 2'I. Line 38 represents the cathode potential variation of tetrode 20 which is used as the output 0f this tube.

To develop the fully reproduced signal shown as line 39, the regenerated signal output of tetrode 3 (line 34) is combined with the output wave of tetrode 2!! (line 38) by means of dual diodev vacuum tube 29. The reproduced dot and dash combination (letter A in the illustrations of Fig. 2) is developed as a voltage across output resistor3, and is shown in line 39.

What is claimed is:

1. In a pulse communication system, a signal regenerator circuit for rebuilding the.received pulses into longer duration pulses, said regenerator circuit including iirst and second self-restoring electron discharge device trigger circuits each having a stable and an active state, each providing a pulse output of substantially rectangular wave form in response to a triggering input pulse, a connection between said trigger circuits for ring said second trigger circuit at the time said first trigger circuit restores itself to its stable state, and means for combining the outputs from both trigger circuits.

2`. In a pulse communication system, a signal regenerator circuit for rebuilding the received pulses into longer duration pulses, said regenerator circuit including rst and second self-` restoring electron discharge device trigger circuits each having a stable and an active state, each providing a pulse output of substantially rectangular wave form in response to a triggering input pulse, a connection between said trigger circuits for the iiring said second trigger circuit at the time said iirst trigger circuit restores itself to the stable state, the time constants of said trigger circuits having such values that said second trigger circuit provides output pulses of a duration sufficient to bridge the gaps or spaces between certain output pulses of said i-lrst trigger circuit, a.rectiier electrode structure in circuit with the output from each trigger circuit, and means for obtaining the combined outputs-from said rectiiier structure.

3. In a pulse communication system, a signal regenerator .circuit for rebuilding the received pulses into longer duration pulses, said regenera; tor circuit including first and second self-restore ing electron discharge device trigger circuits, each providing a -pulse output of substantially rectangular.' wave form; in response' -tc. a triggering Airlllii"pulse; a: connection between said -trigger vcircuits fortrin'gsaid second trigger circuit from. saidzfirst triggercircuit, lsaid 'connection includ,- ing adiierentiator inV circuit therewith for causingsaid secondA trigger circuit tooperate'substantially only when said rst trigger.'A circuit restores itselfto normal and having beenfired, theftme constants ofv said. second trigger cir'- cuitzbein'g such Athat it'provides outputpulses ofA a duration sufcient to bridge thewgaps; or spaces betweenzcertain output pulses.` from said iirst trigger circuit, and means for combining the outputs 1 from both trigger circuits.

4*.xIn1vaipulsef communication system; a signal regenerator circuit for rebuilding the received pulses.V into longer f duration pulses, said "regenerator lcircuit includingfirst and second: selfrestoring electron discharge device triggerrcircuits,` each providing a pulse output of substantiailyf rectangular wave form in response to a triggering input pulse, .a connection between said trigger circuits icr'controlling the operation of said-second trigger circuit from said iirstltrigger circuit, said connection including a diii'erentiatoriin'circuittherewith' for causing `said second trigger circuit to operate substantially onl'ywhen said rst trigger circuit restores itself 4to'normal andv having been fired, the time constants offsaid second trigger circuit being such thatit provides output pulses of la duratiOnSuiiiCient to bridgethe gaps orfspaces between Vcertain output pulsesifrom said rst trigger circuit, anda dual diode coupled to output electrodes of both trigger circuits for combining the output pulses from both trigger circuits;

5. In'a pulse' communication system, al signal regenerator circuitv for rebuilding the received pulses into longer duration pulses, saidregen'- erator circuit including first andsecond selflrestoring electron discharge device'triggerr circuits, each providing a pulse outputv of'substantiallyv rectangular 'wave form in response'to :a triggering inputpulse, a connection betweenLsaid trigger circuits for iiring said second 'trigger circuit attthe time said first 'trigger' circuit restoresfvit= self to normal, said nrst trigger circuitcomprisin'gai pair' of Vinterconnected tetrode devices,n 'a source of unidirectional potential coupled to-'the screen grid of the input tetrode device: of said first trigger circuit for supplying a bias which' determines the operating threshold, and means forr combining the outputsfrom'both triggercircuits;

k6. In' a. pulsevw communication system, 'ai signal regenerator circuit for rebuildingV the received pulses' into longer duration pulses, said -regenerator circuit includingrst andv 'secondtself-restoring gaseous electron discharge device trigger circuits, each providing a pulsev outputA of substantiallyl rectangular 'wave-A form in response to'a'triggering input pulse, each of saiditrigger circuits comprising a'. pair of interconnected electrode structures, a connection,between'said.trigi ger circuits forv ring said second triggercircuit at the vtime said first Atrigger circuit restoresfitself to normal, and rectifier meansor combining thel outputs from both trigger circuits.

7. A pulse telegraph communication; system wherein equal length pulses, each of a time duration shorter than that-of a dot, areitransmitted; aareceiving terminal including ,af regeneratorsys.-v tem=for rebuilding the received pulses; into; longer duration: conventional codef characters, said regenerator;- system comprisingf an: electroni-discharge device-*trigger vcircuit for reshaping ,the received pulses into substantially rectangular shaped pulses, said trigger circuit havingv a stable andan active state and comprising apair of--in. terconnected Vmulti-electrode structures, means in circuit with said trigger circuit for preventing its operation on pulses below a predetermined magnitude, another electron discharge device trigger circuit also having a stable and an active state and coupled to and under control of said first trigger'circuit by virtue of a coupling which causes the second trigger circuit to operate substantially only on the termination ofthe active state of said first triggercircuit, and means for combining the outputs from both trigger circuits in a. common utilization circuit.

8; A pulse telegraph communication system wherein Vequal length pulses, each of a time duration shorter than that of ya dot, are transmitted, acreceivingterminal including a regenerator system for rebuilding the received pulses into longer duration conventional code characters, said regenerator system comprising an electron discharge devicev trigger circuit for reshaping the received pulses into substantially rectangular shaped pulses,'said 4trigger circuit having a stable and an active state and comprising a'pair of'ine terconnected multi-electrode structures, means in circuit with said trigger circuit for'preventing its operation on pulses below a predetermined magnitude, another electron discharge device trigger. circuit also having a stable and an vactive state and coupled to and under controlof said iirsttrigger crcuitby virtue of a couplingwhich causes the second trigger circuit to operate sub;- stantially only on the termination of the active state of said rsttrigger circuit, and means'for combining the outputs from both trigger circuits in a common utilization circuit, the time constants of said second trigger circuit being such that it provides output pulses of a duration shorter than the spacing between said equal length pulses but suiii'cient to ll in the gaps'or spaces between certain output pulses from the first trigger circuit.

9. In a pulse communication system of the typev wherein groups of time spaced relatively shortpulses of given nature are transmitted to represent longer pulses of the same nature, the transmitted short pulses being subject to distortion during transmission, a pulse reproducing cir' cuitarrangement including a monostable multivibrator circuit having a stable and an active state, means to apply received signals to said multivibrator circuit whereby said short pulses-as received are regenerated in essentially the same form as transmitted, a further monostable multivibrator circuit having a stable and an active state and'being coupled to the rst said multivibrator circuit to be activated upon said rst multivibrator circuit being restored to the'stable state, the active period of said f-urther multivi# brator circuit being substantially equal to the timesspacing between the'short pulses of each 'of said' groups as regenerated, and 'a circuit directly connected to both of said multivibrator circuits to'combine the outputs thereof Vto reproduce-said longer pulses.

`10; In a pulse communication system ofithe typewherein groups of time spaced relatively short: pulses ofgiven nature are transmitted' to represent-longer Vpulses vof the same nature, the transmittedishort pulsesy being subj ect to' distortion-during'transmission, apulse reproducingfcirv cuittarrangementfincluding a monostable multi.-

vibrator circuit having a stable and an active State producing a substantially rectangular output waveform, means to apply received signals to said multivibrator circuit whereby said short pulses as received are regenerated in essentially the same form as transmitted, a further monostable multivibrator circuit having a stable and an active state producing a substantially rectangular output waveform and being coupled to the iirst said multivibrator circuit to be activated upon said rst multivibrator circuit being restored to the stable state, the active period of said further multivibrator circuit being substantially equal to the time spacing between the short pulses of each of said groups as regenerated by said first monostable multivibrator circuit, unidirectional current passing devices coupled to said multivibrator circuits and having a common output circuit to combine the outputs from both of said multivibrator circuits to reproduce said longer pulses.

11. In a telegraph communication system of the type wherein groups of time spaced relatively short pulses of given nature and rectangular Waveform are transmitted to represent longer pulses of the same nature and waveform, there being distortion of said transmitted short pulses as received over the medium oi transmission, a pulse reproducing circuit arrangement including a monostable multivibrator circuit having a stable and an active state producing an output of substantially rectangular waveform, means to apply received signals to said multivibrator circuit whereby said short pulses as received are regenerated in essentially the same form as transmitted, a further monostable multivibrator circuit coupled to the iirst said multivibrator circuit to be activated upon said first multivibrator circuit being restored to the stable state, the active period of said further multivibrator circuit being substantially equal to the time spacing between the short pulses of each of said groups as regenerated, and a pair of rectiiier devices having a common output circuit coupled to both of said multivibrator circuits to combine the outputs of said multivibrator circuits to reproduce said longer pulses.

12. In a telegraph communication system of the type wherein groups of time spaced relatively short pulses of given nature and rectangular waveform are transmitted to represent longer pulses of the same nature and wave form, there l0 being distortion of said transmitted short pulses as received over the medium of transmission, a pulse reproducing circuit -arrangement including a monostable multivibrator circuit having a stable and an active state producing an output of substantially rectangular waveform, means to apply received signals to said multivibrator circuit whereby said short pulses as received are regenerated in essentially the same form as transmitte'd, a further monostable multivibrator circuit coupled to the rst said multivibrator circuit to be activated upon said first multivibrator circuit being restored to the stable state, the active period of said further multivibrator circuit being substantially equal to the time spacing between the short pulses of each of said groups as regenerated in the first said circuit, an output load device, and rectifier devices coupling said load device to both of said multivibrator circuits to combine the outputs thereof to reproduce said longer pulses.

WARREN H. BLISS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,926,821 Stansbury Sept. 12, 1933 1,954,028 Stansbury Apr. 10, 1934 2,050,059 Koch Aug. 4, 1936 2,119,389 Hunt May 31, 1938 2,226,706 Cawein Dec. 31, 1940 2,272,070 Reeves Feb. 3, 1942 2,273,193 Heising Feb. 17, 1942 2,286,450 White June 16, 1942 2,299,272 Hallden Oct. 20, 1942 2,331,124 Livingston Oct. 5, 1943 2,365,450 Bliss Dec. 19, 1944 2,372,106 Nagel Mar. 20, 1945 2,385,214 Livingston Sept. 18, 1945 2,402,917 Miller June 25, 1946 2,403,918 Grosdoi July 16, 1946 2,408,794 Matte Oct. 8, 1946 2,432,188 Bliss Dec. 9, 1947 2,441,246 Miller May 11, 1948 FOREIGN PATENTS Number Country Date 357,532 Great Britain Sept. 14, 1931 

